Method of manufacturing rivets having high strength and formability

ABSTRACT

A rivet having improved formability is provided. The rivet has a shank having a head at one end. The shank and the head have a refined grain structure. The rivet is manufactured from the region of the workpiece having a refined grain structure by first forming a region having a refined grain structure in a workpiece and then forming the rivet. The refined grain structure results in improved mechanical properties, such as formability, strength, toughness, ductility, corrosion resistance, and fatigue resistance. The improved formability of the rivet reduces the formation and propagation of cracks during the manufacture and installation of the rivets.

This application is a divisional of Ser. No. 10/145,342 filed May 14,2002.

FIELD OF THE INVENTION

The present invention relates to rivets and, more particularly, relatesto a method of manufacturing rivets having high strength andformability.

BACKGROUND OF THE INVENTION

Structural assemblies are commonly formed by joining two or morestructural members using fasteners, such as rivets. In the aerospaceindustry, where weight and strength are of critical concern, the jointsof structural assemblies typically are subjected to repeated cycles ofshear, compressive, and tensile stresses over the life of the assembly.As a result, the rivets must have good mechanical strength and fatigueresistance without adversely affecting the overall weight of thestructural assemblies. In addition, because the structural assembliesmay be exposed to the ambient environment, including moisture exposureand temperature fluctuations, the joints must be secured with rivetshaving good corrosion resistance and resistance to thermal stresses. Toaddress the strength and weight requirements, conventional rivets aretypically formed of materials having high strength-to-weight ratios,such as aluminum and aluminum alloys that have been hardened by coldworking or precipitation hardening. Advantageously, a number of highstrength aluminum alloys are available that are lightweight, and alsohave relatively high fatigue and corrosion resistance. Unfortunately,when in the hardened condition, high strength aluminum alloys tend tolack the formability that is necessary during manufacture andinstallation of the rivets, which can result in failure by necking,cracking or tearing.

In seeking to solve the problems associated with poor formability,modifications to the manufacturing process for forming the rivets havebeen proposed. One such modification includes forming the rivets from analuminum alloy that is in a soft condition and, thereafter, heattreating the rivet, such as by precipitation hardening, to therebyharden the rivet prior to installation and use. The increase informability of aluminum alloys in a soft condition reduces thelikelihood that the rivet will fail as a result of necking, cracking, ortearing during manufacture. However, heat treating reduces theformability of the rivets which, as noted above, can result in failureduring installation. Heat treating also adds an additional step duringmanufacture, which increases the manufacturing costs of the rivets andresulting structural assemblies.

Accordingly, there exists a need for an improved method formanufacturing rivets. The method should provide rivets having highformability to reduce the likelihood of necking, cracking, or tearingduring the manufacture and installation of the rivets. The method alsoshould be cost effective so as not to adversely affect the manufacturingcost of the rivets and the resulting structural assemblies. In addition,the rivets should be capable of being formed from materials that havehigh strength-to-weight ratios, and that exhibit high fatigue andcorrosion resistance, as well as resistance to thermal stresses.

SUMMARY OF THE INVENTION

The present invention provides a method of manufacturing rivets.According to one embodiment of the present invention, the methodincludes providing a workpiece defining at least one region having arefined grain structure. In one embodiment, the providing step includesdetermining the dimensions of the rivet, selecting the workpiece basedon the dimensions of the rivet, and then friction stir welding a portionof the workpiece to form the at least one region having a refined grainstructure. In another embodiment, the providing step includes insertinga rotating friction stir welding probe into the workpiece to form the atleast one region having a refined grain structure. The rotating frictionstir welding probe can be moved through the workpiece along apredetermined path to form an elongate region having a refined grainstructure. A blank is then removed from the at least one region of theworkpiece having a refined grain structure. In one embodiment, theremoving step includes punching the blank from the at least one regionof the workpiece having a refined grain structure. The blank is thenformed into a rivet. In one embodiment, the workpiece is machined priorto the forming step to remove at least one region of the workpiecehaving an unrefined grain structure. In another embodiment, the formingstep includes extruding the blank through a die. In yet anotherembodiment, the forming step includes stamping the blank with a punch.In still another embodiment, the removing and forming steps arerepeated.

According to another embodiment of the present invention, the method ofmanufacturing rivets includes providing a workpiece. At least one regionhaving a refined grain structure is then formed in the workpiece. In oneembodiment, the at least one region is formed by inserting a rotatingfriction stir welding probe into the workpiece. The rotating frictionstir welding probe can be moved along a predetermined path to form anelongate region having a refined grain structure. Subsequent to thefirst forming step, a rivet is formed from the at least one regionhaving a refined grain structure. In one embodiment, the second formingstep includes removing a blank from the at least one region of theworkpiece having a refined grain structure and forming the blank into arivet. The removing step can include punching the blank from the atleast one region of the workpiece having a refined grain structure. Inanother embodiment, the blank is formed into a rivet by extruding theblank through a die. In another embodiment, the blank is formed into arivet by stamping the blank with a punch. In yet another embodiment, themethod includes machining the workpiece prior to forming the rivet toremove at least one region of the workpiece having an unrefined grainstructure. In another embodiment, the method includes repeating thesecond forming step.

The present invention also provides a rivet having improved formability.The rivet includes a shank that has a head at one end thereof. The shankand head substantially comprise a grain structure having a grain sizeless than about 5 microns. In one embodiment, the shank and headcomprise aluminum, an aluminum alloy, titanium, or a titanium alloy. Inanother embodiment, the end of the shank opposite the head is adapted tobe upset to form a second head.

The present invention also provides a structural assembly including afirst structural member and a second structural member positionedadjacent to the first structural member to thereby define an interfacetherebetween. The structural assembly includes at least one rivet atleast partially joining the first and second structural members alongthe interface. The rivet substantially comprises a refined grainstructure having a grain size less than about 5 microns. The firststructural member and the second structural member can comprise the sameor dissimilar materials. In one embodiment, the first and secondstructural members comprise aluminum, an aluminum alloy, titanium, or atitanium alloy. In another embodiment, the rivets comprise aluminum, analuminum alloy, titanium, or a titanium alloy. In yet anotherembodiment, the structural assembly includes an elongate weld joint atleast partially joining the first and second structural members alongthe interface. In one embodiment, the elongate weld joint at leastpartially consumes at least one of the at least one rivets. In anotherembodiment, the elongate weld joint is an arc weld joint, resistanceweld joint, gas weld joint, or friction stir weld joint.

Accordingly, there has been provided a rivet having improvedformability, and an associated method of manufacturing the same. Themethod of manufacturing allows for the cost effective manufacture ofrivets for forming structural assemblies, including structuralassemblies for aerospace applications. The rivets have improvedformability to reduce necking, cracking, or tearing during themanufacture and installation of the rivets. In addition, the rivets arecapable of being formed from materials that have high strength-to-weightratios and that exhibit high fatigue and corrosion resistance, as wellas resistance to thermal stresses.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention, andthe manner in which the same are accomplished, will become more readilyapparent upon consideration of the following detailed description of theinvention taken in conjunction with the accompanying drawings, whichillustrate preferred and exemplary embodiments, and which are notnecessarily drawn to scale, wherein:

FIG. 1 is an elevation view illustrating a structural assembly thatincludes first and second structural members joined by rivets, accordingto one embodiment of the present invention;

FIG. 2 is a plan view illustrating the structural assembly of FIG. 1;

FIG. 3 is an elevation view illustrating a rivet, according to oneembodiment of the present invention;

FIG. 4 is a plan view illustrating the rivet of FIG. 3;

FIG. 5 is an elevation view illustrating a rivet after the end of therivet has been upset, according to one embodiment the present invention;

FIG. 6 is a perspective view illustrating a friction stir welding probeused to friction stir weld a workpiece to form a region having a refinedgrain structure, according to one embodiment of the present invention;

FIG. 7 is a perspective view illustrating a workpiece having a refinedgrain structure, according to one embodiment of the present invention;

FIG. 8 is a perspective view illustrating a blank, at least a portion ofwhich has a refined grain structure, according to one embodiment of thepresent invention;

FIG. 9 is a black and white photograph illustrating the grain structureof a rivet formed of AA 2017-T4 aluminum alloy at approximately 100times magnification, as is known in the prior art;

FIG. 10 is a black and white photograph illustrating the grain structureof a rivet formed of AA 2017-T4 aluminum alloy at approximately 500times magnification, as is known in the prior art;

FIG. 11 is a black and white photograph illustrating the refined grainstructure of a rivet formed of AA 2195-T6 aluminum alloy atapproximately 100 times magnification, according to one embodiment ofthe present invention;

FIG. 12 is a black and white photograph illustrating the refined grainstructure of a rivet formed of AA 2195-T6 aluminum alloy atapproximately 500 times magnification, according to one embodiment ofthe present invention;

FIG. 13 is an elevation view illustrating a pair of structural membersthat are positioned for joining and that define an aperture forreceiving a rivet, according to one embodiment of the present invention;

FIG. 14 is an elevation view illustrating a rivet inserted into theaperture defined by the structural members of FIG. 13, according to oneembodiment of the present invention;

FIG. 15 is an elevation view illustrating the rivet of FIG. 14positioned in a press, according to one embodiment of the presentinvention;

FIG. 16 is an elevation view illustrating the end of the rivet of FIG.15 partially upset by the press, according to one embodiment of thepresent invention;

FIG. 17 is an elevation view illustrating the end of the rivet of FIG.16 further upset by the press, according to one embodiment of thepresent invention;

FIG. 18 is an elevation view illustrating a structural assembly formedby the joining of the structural members using the rivet of FIG. 17,according to one embodiment of the present invention;

FIG. 19 is a plan view illustrating a structural assembly joined by anelongate weld joint that at least partially consumes at least one of therivets, according to one embodiment of the present invention;

FIG. 20 is a flow chart illustrating the steps for manufacturing rivets,according to one embodiment of the present invention; and

FIG. 21 is a flow chart illustrating the steps for manufacturing rivets,according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, this invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

Referring now to the drawings, and in particular to FIGS. 1 and 2, thereis illustrated a structural assembly 1, according to one embodiment ofthe present invention. The structural assembly 1 includes a firststructural member 2 and a second structural member 3 positioned adjacentthe first structural member 2 so as to define an interface 6therebetween. The structural assembly 1 includes one or more rivets 4joining the first and second structural members 2, 3 together. Thestructural assembly 1 also defines apertures 5, which extend through thefirst and second structural members 2, 3 and are structured to receive acorresponding rivet 4. Although two structural members are illustrated,the number of structural members that can be joined together accordingto the present invention is not limited to two, but can include onestructural member or three or more structural members. For example,according to one embodiment (not shown), the ends of a single structuralmember can be joined together.

Each structural member 2, 3 can be machined, through known manufacturingmeans, from a single workpiece into a predetermined shape and thicknessas required by the specific design loads and specifications of theresulting structural assembly 1. For example, a CNC milling machine canbe used to machine each structural member 2, 3, as necessary. Thestructural members 2, 3 can be manufactured in a variety ofconfigurations, including, for purposes of example only and notlimitation, plates, blocks, tubular members, and curvilinear members.Similarly, the structural members 2, 3 can be formed of a variety ofmaterials, as required by the specific design loads and specificationsof the resulting structural assembly 1. The structural members 2, 3preferably are formed of materials having high strength-to-weightratios, including, for purposes of example only and not limitation,aluminum, aluminum alloys, titanium, titanium alloys, or steel alloys.As illustrated in FIG. 13, the structural members 2, 3 are pre-machinedusing known manufacturing methods, such as drilling or punching, to formapertures 5, each of which is structured to receive a correspondingrivet 4. The dimensions and configuration of each aperture 5 are basedon the dimensions and configuration of the corresponding rivet 4.

Referring to FIGS. 3-5, each rivet 4 has a head 11 and a shank 10extending therefrom. As illustrated in FIG. 14, the shank 10 of eachrivet 4 is structured to extend through the corresponding apertures 5defined by the first and second structural members 2, 3. The head 11 ofthe rivet 4 has a diameter that is larger than the aperture 5 of thefirst structural member 2 through which the shank 10 extends. The end 14of the shank 10 opposite the head 11 is structured to be insertedthrough the corresponding apertures 5 defined by the first and secondstructural members 2, 3 and defines a cavity 13 structured to be upsetto form a second head 12, as illustrated in FIGS. 5 and 18, to therebyat least partially join the first and second structural memberstogether. The number of rivets 4 used to join the structural memberswill depend on the particular design loads and specifications of thestructural assembly 1. For structural assemblies 1 having three or morestructural members, each rivet 4 can join two or more structuralmembers.

The rivets 4 are formed of a metal or metal alloy such that the rivetshave a refined grain structure, and preferably a refined grain structurewith a grain size of less than about 0.0002 inches (approximately 5microns). More preferably, the rivets 4 are formed of a metal or metalalloy such that the rivets consist essentially of, according to oneembodiment, or substantially comprise according to another embodiment, arefined grain structure with a grain size ranging in order of magnitudefrom approximately 0.0001 to approximately 0.0002 inches (approximately3 to 5 microns) and having equiaxed shape. As illustrated in FIG. 6, therefined grain structure is formed by mixing or stirring at least aportion of a workpiece 20 with a non-consumable rotating friction stirwelding probe 19. The workpiece 20 can be a stock piece of material,which is selected based on the number and dimensions of the rivets 4that are to be formed from the workpiece 20 and based on the materialproperty requirements of the rivets. The rivets 4, and thus, theworkpiece 20, can be formed from a variety of materials, as required bythe specific design loads, environmental conditions, and specificationsof the resulting structural assembly 1.

To effect mixing of the workpiece 20, the workpiece is first secured toa worktable of a friction stir welding machine by means of aconventional clamp (not shown). The friction stir welding probe 19 isattached to a rotatable spindle 18 of the friction stir welding machine.The friction stir welding machine can include a device such as a drillor milling machine (not shown), that rotates the spindle 18 to therebyrotate the probe 19, for example in the direction indicated by arrow 24.The rotatable spindle 18 is preferably adapted to move the probe 19parallel to the surface of the workpiece 20, for example in thedirection indicated by arrow 25. As the friction stir welding probe 19is forced through the outer surface of the workpiece 20, friction isgenerated between the probe 19 and the workpiece 20. An opening can bepredrilled or tapped through the outer surface of the workpiece 20 toreceive the rotating probe 19, but preferably the rotating probe 19 isthrust directly into the outer surface of the workpiece. The frictiongenerates sufficient heat energy to plasticize the portions of theworkpiece 20 proximate to the probe 19. More specifically, once insertedinto the workpiece 20, the rotating probe 19 imparts mixing under theshoulder 19 a of the probe 19 by shearing action parallel to the outersurface of the workpiece 20. The rotating probe 19 also imparts mixingaround the threaded portion of the probe 19 parallel to the probe axis.See U.S. Pat. No. 5,460,317 to Thomas et al. for a general discussion offriction stir welding, the entire contents of which are incorporatedherein by reference. The probe 19 can be moved randomly throughout theworkpiece 20 or along a predetermined path that is chosen so as tofriction stir weld or mix a certain region or regions 21 of theworkpiece 20. Upon cooling, the region or regions 21 of the workpiece 20that were mixed by the rotating probe 19 have a refined grain structurehaving improved strength, toughness, ductility, fatigue resistance, andcorrosion resistance so that the material will resist the formation andpropagation of cracks. Thus, there is formed in the workpiece 20 atleast one region 21 of the workpiece 20 that has a refined grainstructure.

As discussed above, the rotating probe 19 can be used to friction stirweld or mix a certain region or regions 21 of the workpiece 20, or, inanother embodiment (not shown), the rotating probe 19 can be used tofriction stir weld or mix all or substantially all of the workpiece 20.For example, the assignee of the present application has developedmethods and apparatuses for refining the grain structure of a workpiece,as disclosed in commonly owned U.S. application Ser. No. 10/609,951entitled “Method and Apparatus For Producing a Refined Grain Structure”filed concurrently herewith, the entire disclosure of which is herebyincorporated by reference.

As illustrated in FIGS. 7 and 8, the region or regions 22 of theworkpiece 20 having an unrefined grain structure can be removed from theworkpiece 20, for example by machining the workpiece 20, to thereby forma blank 23 substantially comprising the region or regions 21 of theworkpiece 20 having a refined grain structure. The blank 23 also can beformed by punching the blank from the region or regions 21 of theworkpiece 20 having a refined grain structure. The rivets 4 can bestamped, punched, extruded, or milled from the workpiece 20 or blank 23,as is known in the art. For example, the rivets 4 can be formed byextruding the blank 23 through a die or stamping the blank 23 with apunch. One blank 23 preferably is used to form a number of rivets 4 sothat the rivets 4 can be manufactured cost effectively.

Referring to FIGS. 11 and 12, there is illustrated at 100 times and 500times magnification, respectively, the refined grain structure of therivets 4 formed according to the present invention from AA 2195-T6aluminum alloy. In contrast, FIGS. 9 and 10 illustrate the grainstructure of conventional rivets formed of AA 2017-T4 aluminum alloy at100 times and 500 times magnification, respectively. Advantageously, therivets 4 formed according to the present invention have a refined grainstructure that resists the formation and propagation of cracks and,thus, have improved formability so as to resist necking, cracking, ortearing during manufacture and installation. While not intending to bebound by any particular theory, it is believed that the refined grainstructure or fine-grain material from which the rivets 4 are formedaccording to the present invention is more formable than the unrefinedgrain structure or course grained material used to form conventionalrivets, since the former has a greater total grain boundary area toimpede dislocation motion. This is contrary to the conventionalrelationship between grain size and formability that results from coldworking, i.e., cold working increases strength and refines grain size,but decreases formability.

Referring to FIGS. 13-18, the structural assembly 1 is constructed bypositioning the first structural member 2 relative to the secondstructural member 3 such that the structural members 2, 3 define aninterface 6 therebetween. An aperture or apertures 5, as illustrated inFIG. 13, are formed in the structural members 2, 3, for example bydrilling. The apertures 5 can be formed before or after positioning thestructural members 2, 3 relative to one another. The number of apertures5 formed in the structural members 2, 3 depends on the number of rivets4 that will be used to join the structural members 2, 3. As illustratedin FIG. 14, the shank 10 of each rivet 4 is inserted into and throughthe corresponding apertures 5 defined by the first and second structuralmembers 2, 3 so that the end 14 of the shank 10 defining the cavity 13extends from the aperture of the second structural member 3.

The end 14 of the shank 10 defining the cavity 13 is deformed in aprocess known as “upsetting.” In one method of upsetting, the end 14 ofthe shank 10 is deformed by applying a compressive force to the shank10. For example, as illustrated by the arrows 16 a, b in FIGS. 15-18, apress 15 can be used to apply a compressive force to the end 14 of theshank 10 and the head 11 of the rivet 4. As shown in FIGS. 16 and 17,the forces applied by the press 15 compress and deform or “upset” theend 14 of the shank 10 and the cavity 13 defined therein. Thedeformation of the end 14 of the shank 10 causes the end 14 to increasein diameter and form a second head 12, as illustrated in FIG. 18. Thediameter of the second head 12, is larger than the diameter of thecorresponding aperture 5 of the second structural member 3 so that therivet 4 joins the first and second structural members 2, 3, thus formingthe structural assembly 1. In other embodiments (not shown), othermethods and devices may be used for upsetting the end 14 of the shank 10of the rivet 4. For example, the rivets 4 can be installed using aconventional rivet gun (not shown).

In addition to the other advantages discussed above, the rivets 4 of thepresent invention also are compatible with other known methods ofjoining structural members 2, 3. For example, as shown in FIG. 19, anelongate weld joint 25 can be formed to at least partially join thestructural members 2, 3 to form the structural assembly 1. The weldjoint 25 can be formed by various methods, including arc welding,resistance welding, gas welding, and friction welding. As shown in FIG.19, the elongate weld joint 25 can consume one or more of the rivets 4.

Where the elongate weld joint 25 comprises a friction stir weld joint,the structural members 2, 3 can be formed of either similar ordissimilar metals. Advantageously, since the structural members 2, 3 arejoined by friction stir welding and rivets 4, the structural members 2,3 can be formed of dissimilar metals that would be unweldable oruneconomical to join by conventional fusion welding techniques.Unweldable materials, when joined by conventional fusion weldingtechniques, produce relatively weak weld joints that tend to crackduring weld solidification. Such materials include aluminum and somealuminum alloys, particularly AA series 2000 and 7000 alloys. The use offriction stir welding and rivets 4 permits the structural members 2, 3formed of unweldable materials to be securely joined. Friction stirwelding and rivets 4 also can be used to securely join weldablematerials to other weldable and to unweldable materials. For example,one or both of the structural members 2, 3 can be formed of aluminum,aluminum alloys, titanium, or titanium alloys. Thus, according to thisembodiment of the present invention, the materials that form thestructural members 2, 3 can be chosen from a wider variety of lightweight, high strength metals and alloys, thereby facilitating reductionof the overall weight of the resulting structural assembly 1, which is acritical concern in the aerospace industry.

Referring now to FIG. 20, there is illustrated the operations performedto manufacture rivets, according to one embodiment of the presentinvention. The method includes providing a workpiece defining at leastone region having a refined grain structure. See block 30. In oneembodiment, the providing step includes determining the dimensions ofthe rivet, selecting a workpiece based on the dimensions of the rivet,and friction stir welding a portion of the workpiece to form the atleast one region having a refined grain structure. See blocks 31, 32,and 33. In another embodiment, the providing step includes inserting arotating friction stir welding probe into the workpiece to form the atleast one region having a refined grain structure. See block 34. Therotating friction stir welding probe can be moved through the workpiecealong a predetermined path. See block 35. The method includes removing ablank from the at least one region of the workpiece having a refinedgrain structure. See block 36. In one embodiment, the workpiece ismachined to remove at least one region of the workpiece having anunrefined grain structure. See block 37. In another embodiment, theblank is punched from at least one region of the workpiece having arefined grain structure. See block 38. The blank is then formed into arivet. See block 39. In one embodiment, the forming step includesextruding the blank through a die. See block 40. In another embodiment,the forming step includes stamping the blank with a punch. See block 41.In still another embodiment, the removing and forming steps are repeatedto form additional rivets. See block 42.

Referring now to FIG. 21, there is illustrated the operations performedto manufacture rivets, according to another embodiment of the presentinvention. The method includes providing a workpiece. See block 50. Atleast one region having a refined grain structure is formed in theworkpiece. See block 51. In one embodiment, the forming step includesinserting a rotating friction stir welding probe into the workpiece. Seeblock 52. The method may also include moving the rotating friction stirwelding probe along a predetermined path. See block 53. In anotherembodiment, the workpiece is machined to remove at least one region ofthe workpiece having an unrefined grain structure. See block 54. A rivetis formed from the at least one region having a refined grain structure.See block 55. In one embodiment, the step of forming the rivet includesremoving a blank from the at least one region of the workpiece having arefined grain structure. See block 56. The removing step may comprisepunching the blank from the at least one region of the workpiece havinga refined grain structure. See block 57. The blank is then formed into arivet. See block 58. In one embodiment, the rivet is formed by extrudingthe blank through a die. See block 59. In another embodiment, the rivetis formed by stamping the blank with punch. See block 60. The forming ofthe rivet can then be repeated to form additional rivets. See block 61.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

1. A structural assembly, comprising: a first structural member; asecond structural member positioned adjacent to said first structuralmember to thereby define an interface therebetween; and at least onerivet at least partially joining said first and second structuralmembers along said interface and wherein said at least one rivetsubstantially comprises a refined grain structure having a grain sizeless than about 5 microns.
 2. A structural assembly according to claim 1wherein said first structural member and said second structural membercomprise dissimilar materials.
 3. A structural assembly according toclaim 1 wherein said first and second structural members comprise thesame material.
 4. A structural assembly according to claim 1 whereinsaid at least one rivet comprises a material selected from the groupconsisting of aluminum, an aluminum alloy, titanium, and a titaniumalloy.
 5. A structural assembly according to claim 1 wherein at leastone of said first and second structural members comprises a materialselected from the group consisting of aluminum, an aluminum alloy,titanium, and a titanium alloy.
 6. A structural assembly according toclaim 1 further comprising an elongate weld joint joining said first andsecond structural members at least partially along said interface.
 7. Astructural assembly according to claim 6 wherein said elongate weldjoint at least partially consumes at least one of said at least onerivets.
 8. A structural assembly according to claim 6 wherein saidelongate weld joint comprises a weld joint selected from the groupconsisting of an arc weld joint, resistance weld joint, gas weld joint,and friction stir weld joint.